The present invention relates to a thermal transfer image receiving sheet and a process for producing such a sheet, and more particularly to a thermal transfer image receiving sheet having a dye receptor layer which is capable of forming a good image by using a thermal transfer system.
The present invention also relates to a thermal transfer sheet which may suitably be used in combination with the above thermal transfer image receiving sheet, is excellent in dye migration property at the time of thermal transfer operation, and is capable of providing a good image having no white defect (or white dropout), etc.
Heretofore, various thermal transfer methods have been known. Among these, there has been proposed a method wherein a sublimable dye (or subliming dye) is used as a recording agent, and is carried on a substrate film such as paper and plastic film to obtain a thermal transfer film, and various full color images are formed on an image receiving sheet such as paper and plastic film having thereon a dye receptor layer, by using the resultant thermal transfer film.
In such a case, a thermal head of a printer is used as heating means so that a large number of color dots of three or four colors are transferred to the image receiving sheet under heating in a very short period of time. As a result, a full color image of an original is reproduced by using the multi-color color dots.
The thus formed images are very clear and are excellent in transparency, since the dyes are used therein as a colorant. Accordingly, these images are excellent in half tone reproducibility and gradation characteristic, and are substantially the same as the images formed by the conventional offset printing and gravure printing. Further, when the above image forming method is used, there can be formed images of high quality which are comparable to full color photographic images.
As the thermal transfer image receiving sheet to be used in the above sublimation type thermal transfer system, there has been used one comprising a substrate sheet and a dye receptor layer disposed thereon. However, since the image receiving sheet is heated at the time of the transfer operation, it causes considerable curl. In addition, in a case where such an image receiving sheet is left standing at a higher or lower temperature before it is used for the thermal transfer operation, it also causes curl, and cannot be fed to a printer in some cases.
As a method for solving such a problem of curl production, there have been proposed a method wherein a back coating layer is bonded to a surface of a substrate sheet reverse to the surface thereof on which a dye receptor layer is formed (Japanese Laid Open Patent Application (JP A, KOKAI) No. 214484/1988), a method wherein an ionization radiation curing adhesive is disposed between a substrate sheet and a dye receptor layer (Japanese Laid Open Patent Application No. 24794/1989), etc. In these methods, however, the problem of curl production has not sufficiently been solved yet.
In the above thermal transfer method, for the purpose of improving the migrating property of the dye, both of the dye layer and dye receptor layer are caused to have a smooth surface so that the thermal transfer sheet may closely contacts the thermal transfer image receiving sheet, and a release agent such as silicone oil is contained in or applied onto the dye layer and/or dye receptor layer so that these layers are not bonded to each other by heat fusion after the printing operation.
Accordingly, the resultant transferred image has excellent surface gloss; but is not suitable in a case where a matted image such as cloth or texture design is desired. Further, when the dye receptor layer is partially transferred to plain paper, etc., to form an image on the dye receptor layer, and ordinary letters, etc., are printed on another portion by another means, the resultant image formed on the dye receptor layer is utterly different from that formed on the plain paper portion. As a result, the entirety of the resultant images appear to be unsuitable or inadequate.
Further, other letters, etc., are written on the above image by using another means such as a pencil, in some cases. In such a case, however, it is difficult to effect writing because of the surface smoothness or the presence of the release agent.
As a method for solving the above problem, there is known a method wherein a so called mat agent (or matting agent) such as kaolin clay, silica, and calcium carbonate is added to the dye receptor layer, as a disclosed in, e.g., Japanese Laid Open Patent Application No. 105689/1987. However, in such a method, a large amount of the mat agent is required in order to reduce the resultant gloss, and therefore the dye receiving property of the dye receptor layer is diminished. As a result, the reproducibility of dots is decreased to cause white dropout or toughening, whereby the resultant image quality is considerably lowered.
Japanese Laid Open Patent Application No. 55190/1990 discloses a method wherein a sheet for regulating (or modifying) the surface condition is pressed to the surface of the thermally transferred image under heating so as to met the image. In this method, however, heating operation is required and there is posed a problem such that the formed dye image is blurred.
Further, in most cases, it is necessary to record information such as letters and symbols, simultaneously with the formation of the above dye image. As a matter of course, such letter information can simultaneously be recorded by using the sublimation type thermal transfer system. However, the resultant letter image formed by such a system is generally unclear because of a limit to the resolution of a thermal head to be used in the above system, and is inferior in image density to black letters provided by other printing means such as heat melting type thermal transfer system, and electrophotographic system.
Accordingly, there has been used a method wherein a gradation image such as photographic image is formed by the sublimation type thermal transfer system and another letter image is formed by other means as described above. In such a case, however, the adhesion property of the other letter images to the receptor layer is poor, so that such images are liable to be peeled by rubbing, etc.
In the above thermal transfer method, for the purpose of improving the migrating property of the dye, both of the dye layer and dye receptor layer are caused to have a smooth surface so that the thermal transfer sheet may closely contacts the thermal transfer image receiving sheet, and a release agent such as silicone oil is contained in or applied onto the dye layer and/or dye receptor layer so that these layers are not bonded to each other by heat fusion after the printing operation.
In such a case, however, there is posed a problem such that the adhesion property between the thermal transfer sheet and dye receptor layer is poor, and therefore the migration of the dye is obstructed, and white dropout, image defect, etc., are produced in the resultant image.
Such a problem may also be posed in the same manner, in a case where post card paper, plain paper, etc., having rough texture are used as the substrate sheet of the image receiving sheet, in place of the mat image receiving sheet, and minute defects are present in the dye receptor layer on the basis of the above rough texture of the substrate.